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Answer: Languages like C# and Java use nominal typing instead.

Example

interface Pet { name: string }
class Dog { name = "Rex"; bark() {} }
let p: Pet = new Dog(); // OK: Dog has name

Rules of thumb

• Return types: the source must return at least what the target promises.
• Private and protected class members make compatibility depend on the declaring class.

Building types from types

Because compatibility is about shape, TypeScript offers operators to derive shapes: keyof, typeof, indexed access, conditional types, mapped types and template literal types.

type Keys = keyof Pet;                 // "name"
type ReadonlyPet = { readonly [K in keyof Pet]: Pet[K] };

Claims

• id: C1 claim: Structural typing relates types based solely on their members. source: S1 evidence: "Structural typing is a way of relating types based solely on their members."
• id: C2 claim: This is in contrast with nominal typing. source: S1 evidence: "This is in contrast with nominal typing."
• id: C3 claim: In C# or Java the equivalent code would be an error. source: S1 evidence: "In nominally-typed languages like C# or Java, the equivalent code would be an error"
• id: C4 claim: TypeScript offers mapped types. source: S2 evidence: "Mapped Types"
• id: C5 claim: TypeScript offers conditional types. source: S2 evidence: "Conditional Types"

Sources

• [S1] TypeScript: Documentation - Type Compatibility. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/type-compatibility.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.
• [S2] TypeScript: Documentation - Creating Types from Types. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/types-from-types.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_20z5qj5r,
  author = {{iinventedlight}},
  title = {{Type compatibility and structural typing}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-type-compatibility/},
  note = {Permanent ID iil:note:20z5qj5r. License: CC-BY-4.0.},
}

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